The invention relates generally to DC--DC converters, and in particular to DC--DC converters which provide high power density.
A DC--DC converter is a device which converts a DC voltage at one level to a DC voltage at another level. The converter typically includes a transformer, having primary and secondary windings wound around a common magnetic core. By opening and closing the primary circuit for appropriate intervals, control over the energy transfer between primary and secondary is accomplished.
Common DC--DC converter topologies include the forward converter and the flyback converter. When the primary winding of the forward converter is energized by closing the primary circuit, energy is immediately transferred to the secondary winding. Energy is accumulated in the transformer of the flyback converter by closing the primary circuit and then released from the secondary winding by opening the primary circuit.
Recently, in the field of DC--DC converters, greater attention has been directed toward compactness, energy efficiency, and higher performance. One prior art method for improving power density of a DC--DC converter is to increase the switching frequency. This results in a reduction of the size of the isolation transformer and the output filter. However, the power dissipation losses during switching events become more significant as the switching frequency increases because the duration of each switching even comprises a larger fraction of each switching cycle duration as the switching frequency increases. To reduce or eliminate switching losses, zero voltage switching techniques have been proposed as described in U.S. Pat. No. 4,959,764 issued to John Bassett for "DC/DC Converter Switching At Zero Voltage" and in U.S. Pat. No. 5,173,846 issued to David Smith for "Zero Voltage Switching Power Converter".
Another prior art method for improving power density of a DC--DC converter is to combine various DC--DC converter topologies to increase the amount of power transferred through the converter and reduce the size of the output filter. Examples of such method can be found in U.S. Pat. No. 4,618,919 issued to Hubert Martin, Jr. for "Topology For Miniature Power Supply With Low Voltage And Low Ripple Requirements" and in U.S. Pat. No. 4,734,839 issued to Fred Barthold for "Source Volt-Ampere/Load Volt-Ampere Differential Converter"
Finally, in U.S. Pat. No. 5,291,382, Isaac Cohen introduces a topology which offers zero voltage switching capability and reduces the size of output filter. However, a MOSFET implementation of the switches of this topology would require the use of a high side driver.